Advice on field coil drivers

What type of box should I consider for this type of driver?

  • The green steel one in the alley...

    Votes: 0 0.0%
  • EV Aristocrat is a suitable enclosure.

    Votes: 1 100.0%
  • Another type of enclosure... Please comment.

    Votes: 0 0.0%

  • Total voters
    1

trainbuftony

Electron Herder
Subscriber
I horse traded into one of these field coil drivers when I met with another tube salvager like myself. At that point I knew nothing about what I had, other than obviously it was a field coil type. It looks outrageously beefy to me. The electro-magnet assembly is quite a bit larger than any of the console pull field coil drivers i have seen. The EIA code is 232 which makes it a Magnavox with a little searching I found out that it came from an organ or an organ tone cabinet. The field coil is 6000 ohms and schematics which I found show it in parallel with the power amp, not in series, like a choke. I decided that this speaker was interesting enough that I had better order another. I found one very affordably, and made it mine via the miracle that is online shopping...

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I had a case of 9v batteries in the shop, so I wired up a field supply by clipping a dozen of them in series, and connecting them to the field coil using jumpers. This allowed me to test the speaker to verify functionality, which I did, and it functions. Not likely to blow anything up with 9v batteries... The voice coil measured 6.3 ohms DC resistance so I felt safe treating it like any other 8 ohm coil. In free air it makes noise, which is better than the alternative. I decided, rather than building boxes for these to stuff them in my EV aristocrat cabinets just to get a sense of how they perform and weather or not I want to continue messing with them. I raided my parts stash and started on building a real power supply in my head. I started by finding some octal tube sockets which I can screw to the inside of my speaker cabinets, and use as connectors for not only the voice coil but for the field coils. I hoard organ parts so finding the pigtail took 30 seconds. I soldered in speaker wires to pins 1 and 5. I have not actually started to build a plug in the wall type power supply, but it should be fairly simple. The field coils want to see about 300 volts across them, Im pretty sure I have a rectifier tube and transformer which will do that... I have read just enough about field coils to be very curious. There are people claiming that they (field coil type drivers) are vastly superior to everything else, due to the (potential) increase in magnetic flux density in the coil gap. Others rave about "tripping over the thick sound". Needless to say I am rather intrigued. I do however want to maintain an open mind. I have less than $100 wrapped up in this project so I have essentially nothing to lose, everything to gain. I want to get a feel for these drivers and potentially build boxes for them at some point. Does anyone have any advice before I start? Am I wasting my time? Is the EV aristocrat corner horn a fair cabinet for this type of driver or should I try something else too?
 
Likely they are not suitable as a full range speaker, obviously, so then the question is, what else do you need to make a complete speaker?

I'd get them tested using WT3 or whatever the new gadget is from Parts Express these days...get the T/S params and go from there.
 
Probably the original cabinet had an open back or maybe a vent. Field coils pretty much disappeared after the 30s so we're not talking hi fi, or sealed or ported cabinets.
 
I'm probably going to work on building the power supply today. If anyone else wants to chime in I'd appreciate any and all advice and or encouragement...
 
I hooked one of the drivers up today, to an organ amp I have. Put 325 volts across the field coil, and put some tones through it. At 5 hz, I could watch the tubes switching as the blue glow alternated between tubes, pretty cool. I noticed the efficiency goes way up when you increase the field strength as expected. Next we them in boxes. I am going to use a spare amp to provide field current with the power tubes removed for testing the new enclosure..
 
I hooked one of the drivers up today, to an organ amp I have. Put 325 volts across the field coil, and put some tones through it. At 5 hz, I could watch the tubes switching as the blue glow alternated between tubes, pretty cool. I noticed the efficiency goes way up when you increase the field strength as expected. Next we them in boxes. I am going to use a spare amp to provide field current with the power tubes removed for testing the new enclosure..


What ever came of this project? I’m curious what your impressions of Fc drivers were.
Thanks,

j
 
Feild coil drivers are the best in terms of efficiency. Find a matched pair with the biggest motor assembly you can... The magnetic Flux which is created in the voice coil gap is far greater than even neodymium magnets can muster.
If you are into making lots of sound with only a little power they do really really well at that.
In my experience they also match well with big boxes specifically back loaded horns.
 
Feild coil drivers are the best in terms of efficiency. Find a matched pair with the biggest motor assembly you can... The magnetic Flux which is created in the voice coil gap is far greater than even neodymium magnets can muster.
If you are into making lots of sound with only a little power they do really really well at that.
In my experience they also match well with big boxes specifically back loaded horns.

Thanks for your reply! Those were my thoughts exactly. I have several pairs of the RCA Victor RE-45 drivers and four pairs of Rola 12” drivers. I will likely use them in OB or H baffles for LF.
My amp of choice is a 3watt Apex tape recorder amp that uses 12ax7 tubes. I cannot find any specific data apart from sensitivity and the ability to alter the Q of the driver to account for the unique sound signature. A friend of mine has done an exhaustive study of FC drivers and believes that there is no difference. He is very knowledgeable and open to myriad approaches to audio including those driven by nostalgia. If you have any input in this regard I’d be very grateful. Thanks again J
 
Point taken. I’d like to see whatever implantation you used with the drivers you have as well as the PSU that you settled on.
 
Feild coil drivers are the best in terms of efficiency. Find a matched pair with the biggest motor assembly you can... The magnetic Flux which is created in the voice coil gap is far greater than even neodymium magnets can muster.
If you are into making lots of sound with only a little power they do really really well at that.
In my experience they also match well with big boxes specifically back loaded horns.
At the time there were prevalent, a five watt amp was "big", and a twenty watt amp was a monster, they had to be efficient.
 
Thanks for your reply! Those were my thoughts exactly. I have several pairs of the RCA Victor RE-45 drivers and four pairs of Rola 12” drivers. I will likely use them in OB or H baffles for LF.
My amp of choice is a 3watt Apex tape recorder amp that uses 12ax7 tubes. I cannot find any specific data apart from sensitivity and the ability to alter the Q of the driver to account for the unique sound signature. A friend of mine has done an exhaustive study of FC drivers and believes that there is no difference. He is very knowledgeable and open to myriad approaches to audio including those driven by nostalgia. If you have any input in this regard I’d be very grateful. Thanks again J
I have a nice RE-45 I'd like to get rebuilt with realistic expectations, but it probably won't happen anytime soon. I will grab an extra loose good amp chassis for it if one turns up.
Fascinating to think Victor sold more than a hundred thousand of these, and I don't know how many R-32, identical without the Electrola record player, which I once had some 60 years past.
Crazy tuning mech that worked very well as a sensitive stable AM radio.
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My "implementation" of field coil drivers is a pair of mismatched Electro-Voice Aristocrat back loaded "corner" horns. I use a Jensen RP-302 super tweeter to cover the top end.
 
A thought I've had about field coil speakers powered from an AB amp, you'd actually have a variable efficiency thing going on. Since the current through the coil would vary with amplifier output, the flux strength would go with it, which should also change speaker efficiency. In effect you'd have something of a dynamic expander sort of effect. Power goes up, and speaker efficiency goes up with it, so net effect would be more sound output at a given power level than you'd get from even the same driver powered off it's own steady-state supply.

Might be an interesting sonic effect, though of course it means designing an amp's power supply around the specific speaker. Not exactly the most flexible arrangement. Also wouldn't really matter if it was a class A amp since the current demands are pretty nearly the same from idle to full scream.
 
The electro-dynamic field coil loudspeaker is a fortuitous evolution of a number of factors, the perfection of the horn-less baffled cone loudspeaker concept in the mid-latter 1920s and the requirement for more audio power for it from the AC powered amplifiers developed for them, the field coil being one of the large power supply smoothing chokes necessary due to the limited capacity of the capacitors of the time combined with the limited power of "permanent" magnets prior to the development of Alnico alloys.
Jensen and Pridham ("Magnavox") developed the field coil magnet moving coil driven diaphragm +horn in the 19##-teens, Rice & Kellogg @ GE/RCA combined an open light paper cone instead of the diaphragm with the Magnavox arrangement and put it in a back-wave cancelling open baffle box enclosure in the mid-1920s. Fundamentally what is still standard practice.

Some battery operated "farm radios" prior to Alnico had large soft iron field magnets suitable for their low audio output power levels.
 
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My "implementation" of field coil drivers is a pair of mismatched Electro-Voice Aristocrat back loaded "corner" horns. I use a Jensen RP-302 super tweeter to cover the top end.
What xover point do you use to the tweeter? How’s the low end? My 9” RCA’s free air are super punchy. I saved the original cones and replaced with DIY wood cones made by a friend in Vietnam:
 
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